It is known that operating voltage surges in atmospheric air are not proportional to the degree of isolation. This is why, at ultra high tension (voltage of not less than 1,000 kV, referred to below as UHT), extremely low operating voltage surges are required for circuit-breakers, e.g. 1.6 pu instead of 3 pu which is acceptable at 245 kV.
The January 1989 edition of the CIGRE review Electra (No. 122, pages 43 and 44) states that UHT circuit-breakers are to be equipped with closing resistors and with opening resistors.
French "Certificate of Addition" No. 2,503,448 (U.S. Pat. No. 4,421,962 ) filed on Mar. 31, 1981 by the Applicant describes a compressed-gas high-tension circuit breaker including two closing resistors whose insertion devices are secured to each other, the insertion device of the first resistor comprising a tubular semi-moving contact that co-operates with a contact connected to the second resistor which is constituted by a stack of annular resistors disposed coaxially about the first resistor.
French Patent Application No. 2,658,949 filed on Feb. 27, 1990 by the Applicant discloses a high-tension circuit-breaker assisted by a varistor disposed in an "auxiliary" chamber which is distinct from the interrupting chamber, and which is connected in series with a break switch comprising a moving contact in the form of a metal rod fixed to a flexible metal cable interconnecting the two chambers via a slow-release opening device situated outside the chambers and actuated by the control rod of the interrupting chamber.
An object of the present invention is to combine, inter alia, those two known techniques so as to provide a UHT circuit-breaker which satisfies the voltage-surge requirements and which is also compact in construction.